Hydraulically-actuated unit fuel injectors are well known in the art and three types having different control valves are described in U.S. Pat. Nos. 5,181,494, 5,460,329, and 5,682,858. Each of these injectors incorporates an electronically-controlled control valve which operates on a hydraulic actuating fluid, such as engine lubricating oil, to operate a hydraulic intensifier piston. Fuel is introduced at a relatively low pressure to the high pressure side of the intensifier piston and, upon pressurization by the intensifier piston to a relatively very high pressure, is delivered to an injection nozzle portion of the injector. The very high pressure fuel lifts a needle valve or check valve from its seat against the closing force of a valve spring to open an injection orifice, thereby causing injection of a charge of fuel into an engine cylinder.
In general, the hydraulic actuating pressure can be varied while the quantity of fuel injected is controlled by the duration during which the actuating pressure is maintained on the intensifier piston. In U.S. Pat. No. 5,682,858, the force of the hydraulic actuating fluid also acts at times against the upper side of the needle check valve to control its opening and closing and thereby provide direct control of the needle check valve.
An important consideration for optimizing diesel exhaust emissions and reducing engine combustion noise is the ability to control the shape of the fuel injection curves, and/or fuel pressure vs. injection duration. This is known in the art as rate shaping. U.S. Pat. Nos. 5,181,494, 5,460,329, and 5,682,858 discuss a number of approaches to rate shaping.
Through rate shaping, the fuel injection curve can be tailored to provide small quantities of fuel during the initial portion of the injection event, control of a single fuel injection event, i.e., single shot injection, or may even cause a split shot injection event having a small pilot injection immediately followed by the large main quantity injection. It is believed that under certain conditions, producing split injection can be very beneficial to reduce overall engine emissions and to reduce diesel engine noise level. However, this small quantity of pilot injection has proved to be very difficult to achieve without introducing significant variability from one injection event cycle to the next event cycle. Pilot injection quantity variability from one injector to the next exists due to machining tolerances and partial motion of the needle check and control valve in the fuel injector. This variability is magnified when the injector is under high actuating pressure. High engine speed operation also magnifies the variability of the pilot injection.
A major cause of the difficulty with pilot injection is that the injector must be configured to provide an orifice of sufficient size for the large fuel quantities supplied for full load engine operation whereas pilot injection might utilize as little as one percent of the full load quantity. When this full size injector is used to inject a very small quantity, the relative variation of the orifice size between the opening for the pilot quantity and the opening needed for full load operation is great and controllability is therefore relatively poor. Since a fully open needle valve can flow too much fuel for small delivery under high injection pressure, it would be desirable to repeatably control the needle lift to effect a very small orifice size and thus greatly limit the fuel quantity delivered during pilot injection.
Under high speed and high load engine conditions, i.e., large quantities of fuel, a single shot, rate-shaped injection may be quite favorable compared to split injection while a split shot or pilot injection may be preferred at lower end of engine operation when low quantities of fuel are required.